Blender GreasePencil fill problems
As per Antonio Vazquez, Blender Developpers, the issue was the leak size. 

It should be defined as 3px, in my case it was 35. 

I was also advised to download the new version of yesterday. 

It works now. 

FYI, the architecture was slightly modified, the brushes and specifications don't appear anymore on the top but on Active Tool and Workspace settings tab, under Brush.